This invention relates to the regeneration of supported silver catalysts. More particularly, the invention relates to an improved method for drying such catalysts which have been regenerated by treating them with solutions of one or more alkali metal, especially cesium and/or rubidium, salts.
Supported silver-based catalysts have been used industrially for many years for the oxidation of ethylene to ethylene oxide with oxygen or air. Most of the ethylene which is reacted is converted into ethylene oxide on the silver-impregnated catalyst support material and the remainder of the ethylene is converted almost exclusively to carbon dioxide and water. The goal is to react as much ethylene as is possible, i.e. high productivity, such that the greater amount of the ethylene is converted to ethylene oxide, i.e. high selectivity.
It is known that the selectivity of these catalysts tends to decrease after they have been used for a number of years. This decrease in selectivity results in less favorable economy of operation and may become of such magnitude as to render further operation with deteriorated catalysts totally uneconomical. Deteriorated catalysts may be replaced with fresh catalysts or may be regenerated. It is advantageous to replace or regenerate catalysts since an increase in selectivity of as little as one percentage point (selectivity=100.times.the amount of ethylene converted to ethylene oxide divided by the total ethylene consumed) will result in the savings of many thousands of dollars in a commercial operation. Regeneration is preferable to replacement since the former is significantly less expensive and less time consuming.
There are several known procedures for regenerating or reactivating used silver-based catalysts. U.S. Pat. No. 4,051,068 issued Sept. 27, 1977, U.S. Pat. No. 4,123,385 issued Oct. 31, 1978, U.S. Pat. No. 4,125,480 issued Nov. 14, 1978, and U.S. Pat. No. 4,177,169 issued Dec. 4, 1979 describe four such methods for regenerating silver catalysts. All four methods involve applying to the spent or used catalysts a regenerant such as a cesium or rubidium compound such that the concentration of the regenerant is in the range of 1 to 1,000 parts per million. This is generally achieved by contacting the used catalyst with a solution of the regenerant in an inert solvent. In the first, second and fourth patents, drying is accomplished by evaporation at 50.degree. to 100.degree. C. although lower temperatures can be used if a vacuum is applied to the catalyst. They all say that this evaporation process can optionally be accomplished while blowing nitrogen through the catalyst. The third patent accomplishes drying by heating at 50.degree. to 200.degree. C. or, preferably, purging the catalyst with a gas stream at 15.degree. to 200.degree. C. None of these patents mention upflow drying or the increase in productivity which can be obtained if upflow drying is used.